The fourth Democratic presidential primary debate will be held on Oct. 15 and 16 in Ohio, according to a memo the Democratic National Committee sent to presidential campaigns.

The memo, which was obtained by POLITICO, says the second night will only be held if necessary. The DNC has previously said it will not have more than 10 candidates on stage at a time.

Candidates can qualify for the debate through Oct. 1, the memo read. The October debates have the same qualifying rules as the September debate: Candidates need to reach 2 percent in four-DNC approved polls and receive donations from 130,000 unique donors. Candidates who qualified for the September debate automatically qualify for the October one, and candidates who missed out have an additional month to meet the requirements, meaning the stage is expected to grow.

The October debate timing was first reported by The New York Times.

The memo said more information on media partners and specific location will be announced "in the coming days."

Ten candidates crossed both of those thresholds and will be on stage for the Sept. 12 debate in Houston: Joe Biden, Cory Booker, Pete Buttigieg, Julián Castro, Kamala Harris, Amy Klobuchar, Beto O’Rourke, Bernie Sanders, Elizabeth Warren and Andrew Yang. It is hosted by ABC News and Univision.

All ten are expected to participate in the October debate, unless they drop out of the race before then. Hoping to join them are billionaire Tom Steyer and Rep. Tulsi Gabbard of Hawaii, both of whom narrowly missed out on the September debate thresholds. Both have crossed the donor mark and need one and two polls, respectively, to make it up on stage in October.

Author Marianne Williamson, who has crossed the donor threshold and has one qualifying poll so far, is also in the hunt to qualify for the mid-October debate.